THURSDAY, May 7, 2015 (HealthDay News) -- Americans' levels of triglycerides -- a type of fat in the blood -- have dropped significantly in the past decade, according to a new federal study.

Factors that may lower triglyceride levels include quitting smoking, weight loss, use of cholesterol-lowering drugs (such as statins) and improving dietary nutrition, the researchers said.

"The news is encouraging," said Dr. Michael Miller, a professor at the University of Maryland School of Medicine in Baltimore.

But Miller, who was not involved with the study, added that it is "certainly nothing to brag about when one out of every three middle-aged men 40 to 59, and nearly one out of every three women aged 60 and over continue to live with unhealthy triglyceride levels."

Like the bad type of cholesterol -- LDL -- high levels of triglycerides raise the risk of clogged arteries and heart disease.

Another expert, Dr. Gregg Fonarow, professor of cardiology at the University of California, Los Angeles, said that "studies have shown that individuals with elevated triglyceride levels are at increased risk for heart attacks, strokes and premature cardiovascular death."

Normally, Miller said, triglycerides are used for energy and storage of excess calories in the body. High levels of triglycerides have also been linked to type 2 diabetes and fatty liver disease, according to the American Heart Association.

The new report analyzed triglyceride levels in the United States from 2001 to 2012. The findings were published by the U.S. Centers for Disease Control and Prevention's National Center for Health Statistics (NCHS) in the May issue of the NCHS Data Brief.

Margaret Carroll and colleagues at the NCHS found that the percentage of adults with triglyceride levels considered to be unhealthy -- 150 milligrams per deciliter of blood (mg/dL) or above -- declined from 33 percent in 2001-2004 to 25 percent in 2009-2012.

There was an especially big dip in people aged 60 and over. Among men over 60, those with elevated numbers dropped from 40 percent in 2001-2004 to 25 percent in 2009-2012. In women over 60, 43.5 percent had high triglycerides in 2001-2004, and that went down to 31 percent for 2009-2012, the study revealed.

Triglyceride levels declined in overweight and obese men and women, the study said.

In Mexican-American women, levels of elevated triglyceride levels dropped from 40 to 28 percent during the study period. However, there were no significant improvements in elevated levels of triglycerides seen in Mexican-American men.

There were also no significant drops in the blood fat levels among black Americans. But, the percentage of blacks with high triglyceride levels was consistently lower than for whites and Mexican-Americans, the report found.

The overall drops in U.S. triglyceride levels are surprising, Fonarow said. "It may have been expected that triglyceride levels should have risen for the U.S. population given the increases in obesity and diabetes," he said.

The study doesn't speculate about the causes of the shift in triglyceride levels. However, Fonarow said, "this is likely due to significant declines in the number of people smoking, as well as improvements in the use of statin medications." Statins are cholesterol-lowering medications that can also lower triglyceride levels, according to the study authors.

Fonarow cautioned that researchers also haven't proven that lowering triglycerides -- but not "bad" LDL cholesterol -- will lower the risk of heart problems. "Further studies are ongoing evaluating the effects of medications that predominately lower triglyceride levels to determine if they provide incremental benefit to statin medications," he said.

Whatever the case, "there are a number of very effective lifestyle modifications that can lower triglyceride levels, including regular exercise, a diet rich in monounsaturated fats, and consuming omega-3 fatty acids." Medications can also be helpful, he said.